Update copyright PR instructions in README

The [README instructions](https://github.com/elementary/wallpapers/blob/master/README.md) for submitting PRs with new wallpapers says to edit the `debian/copyright` file, however this file isn't in the `master` branch (I later found it in the [`deb-packaging`](https://github.com/elementary/wallpapers/blob/deb-packaging/debian/copyright) branch). These instructions should be updated to reflect this (and make it easier for new contributors to submit PRs).

@cassidyjames said of this in https://github.com/elementary/wallpapers/pull/99#issuecomment-613658604:

> Ah, yeah we need to update the instructions for copyright info (or auto-build that file in CI or something). It's in a different branch, but that requires another PR to update.